Aluminum alloys are widely used in a number of commercial and industrial products due to their low density, moderate strength, and resulting high strength to weight ratio, and their environmental resistance. Aluminum alloys may also be used as a matrix for a composite material or article. Alloy utilization is typically limited to component applications with operating temperatures below about 300° F. Aluminum and its alloys are used extensively in the aircraft industry and are also used in other industries such as automotive. Aluminum alloy articles are fabricated by any number of techniques, as may be appropriate for the nature of the alloy and the article. The reduction of aluminum from aluminum-containing bauxite ore is accomplished predominantly by first purifying the ore and subsequently electrolytically reducing it in a cryolite bath by the Hall-Heroult Process. Other reduction methods may also be possible but are not currently in widespread commercial practice. Aluminum metal is produced which can be subsequently used to make aluminum alloys and articles. Some alloying elements and impurities such as iron, silicon, zinc, gallium, titanium, and vanadium may also be present as a result of ore quality and processing and these elements contribute to the final alloy content. Elements and combinations of elements may take many intermediate forms before being melted to form the final aluminum alloy. Alloying content in aluminum alloys may originate from ore reduction processes, virgin additions, or from reclamation of recycled material.
The metal is refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining steps may be performed at the ore stage, during the initial melting process or during remelting operations. After an aluminum alloy of the desired composition is produced, it may be used in the as-cast form for some alloy compositions (i.e. cast alloys), it may be further worked to form the metal to the desired shape for other alloy compositions (i.e. wrought alloys), or it may be atomized to form fine powder and subsequently consolidated and, in some case further worked (i.e. powder metallurgy alloys), or may be solidified rapidly in a powder, ribbon, flake, spray-formed ingot, or other form and subsequently consolidated (i.e. rapidly solidified alloys). Powder alloys may be further modified via mechanically alloying methods. In any case, further processing such as heat treating, machining, surface coating, and the like may be employed.
Regardless of processing route, all of these forms involve melt processing and are, as a result, subject to restrictions imposed by such processes. The melting of aluminum materials may include multiple melt processes to cast the final ingot or article or to produce other intermediate product forms such as powder, ribbon, or flake. In multiple melt processes, intermediate cast electrodes are produced which serve as the input stock to the subsequent melting step. Typical melting processes include various crucible and open hearth melting vessels which can be heated using induction heating, gas or oil firing, electric resistance heating, or electrical radiation heating. Molten aluminum or aluminum alloy is further processed and refined in order to reduce residual hydrogen content, reduce trace element contaminant levels, and establish desired alloying element content. Refining processes may include hydrogen degassing, fluxing, and filtering. Degassing may be accomplished through use of a purge or injected gas which may include reactive and inert gas mixtures, through vacuum degassing, or through other techniques which promote formation and evolution of hydrogen gas bubbles from the molten aluminum alloy. During fluxing operations, inorganic salts can be added which supply anions for reaction with undesirable molten metal contaminants. The flux may also contain active ingredients intended to alloy with the molten metal. The flux layer or injected flux may also act to collect reaction products and contaminants and reduce volatization of high vapor pressure alloying elements and limit the formation of oxide films during melting. Other melt additives such as chlorides or fluorides (aluminum fluoride, for example) may also be used to reduce alkali metal impurities. Filtration of the molten alloy is also used in order to remove solid impurities and is typically accomplished using porous ceramic filters, metal screens, or fiberglass cloth filters.
Composition limitations may be imposed as a result of the melting process for aluminum alloys. Elements with high vapor pressures, significant reactivity, or limited solubility may be desirable due to their contributions to alloy strength, temperature capability, environmental resistance, formability and density. Alloying element content is limited significantly and/or is difficult to control as a result of the melting operations.
Irregularities may result from melting processes or as a result of subsequent forming operations. Melt-related irregularities include those related to segregation as well as those resulting from extrinsic contaminants. Melting of aluminum is subject to the formation of inclusions such as oxides and spinels of aluminum and/or magnesium for example, and highly stable compounds such as borides, carbides, nitrides, and intermetallics, which form from elements such as aluminum, titanium, vanadium, zirconium, manganese, and iron. Liquid phase inclusions such as magnesium chloride may also be problematic. These melt-related inclusions and other irregularities can significantly degrade the performance of cast alloys. Melt-related irregularities can also contribute to forging related irregularities such as cracking, etc. Some materials are also more difficult to form as a result of inheriting coarse cast structures, which can lead to additional forging-related irregularities.
Some aluminum alloys are also produced using powder metallurgy processes to enable production of higher alloying element compositions with resulting increases in alloy strength level. Other rapid solidification processes are also used in order to extend the solid solubility of various alloying element additions through very rapid, non-equilibrium solidification of finely divided material. The current powder metallurgy processes and other rapid solidification processes, however, require alloy material to first be melted and then atomized to produce powder. These powder metallurgy processes add great expense and can still result in extrinsic contamination from crucible ceramics and slag. In addition, powder metallurgy processes are also subject to issues related to gas entrapment in powder particles during the atomization process, which can lead to residual porosity in the resulting billet or component.
As a result, melting processes impose significant limitations on the resulting article. Incremental performance improvement resulting from processing modifications and incremental improvements in production cost reduction are still possible in a number of areas. However, the present inventors have recognized in the work leading to the present invention that in other instances the fabrication approach involving multiple melt steps imposes fundamental performance limitations that cannot be overcome at any reasonable cost. They have recognized a need for a departure from the conventional thinking in fabrication technology towards a meltless process, which will overcome many of these fundamental limitations. The present invention fulfils this need, and further provides related advantages.